7S3 CONDITIONS of FORMATION of LIZARDITE. CHRYSOTILE AND

7s3

The Carndian M irc ralag i st Vol. 33, pp.753-773 (1995)

CONDITIONSOF FORMATIONOF LIZARDITE. CHRYSOTILEAND ANTIGORITE, CASSIAR.BRITISH COLUMBIA

DAVID S. O'HANLEYTeNpFREDERICKJ. WICKS Depamnentof Minzralogy,Royal Ontario Museam, Toronto, Owario MsS 2C6

ABSTRACT

Observationsat the Cassiar(British Columbia) cbrysotile asbestosdeposit have defined a continuousseries of serpentine textures,between pseudomorphic and nonpseudomorphic.These minerals and textures are distributed with respectto shear zonesin the interior of the serpentinitesuch that the degreeof recrystallizationand replacement increases as the shearzones are approached.Pafierns of spatial distribution suggestthat recrystallizationand replacementwere causedby infiltration-driven metamorphismas the serpentiniteequilibrated with an externallyderived fluid. The shearzones served as conduitsforthe fluid. The recrystallizationand replacenentof lizardite formed after olivine and of that formed after enstatiteproceed independently early in ihe process,such tiat compositionof tle variols serpentineminerals was controlled by the bulk compositionof the precursor.fitrt io the process,Fet*/[rlg, CrlAl and Fe3+(Mg + Si) values,and the distribution of boron, betweenserpentine after olivine and that formed after enstatiteindicate that equilibrium was closely approached.The formation of a chrysotile + antigorite assemblagemarks this transition. The recrystallizationof lizardite and its replacementby.chrysotile + antigorite ocqined a1250t 25'C and a P(H2O)of lessthan 1 kbar, The temperaturesand pressuresof serpentinizationcal b9 modeled in the system MgO-SiO2-H2O 6MSII because:l) clinochlore is stabilized by the breakdown of cbromite, rather than a serpentinemineral, and 2) nagnetite is both a product and a reactaatin the conversionoflizaxdite to antigorite,indicating that it is not an essentialcompound in the conversionof lizardite to antigorite.The mineralogy,textures and compositions of lizardite and cbrysotileindicate that they are polymorphsin the MSH system.

Keywords:serpentinite, serpentine, recrystallization, replacement, infltration metamorphism,mindal 3ta6ility, Cassiar'British Columbia.

SomuarnB

Nohe 6tudedu gisementdremiante h chrysotilede Cassiar,en Colombie-Britannique,a permisde ddfinir une s6riecontinue de textures,allant de pseudomorphiqued non pseudomorphique,impliquant les min6raux du groupe de la serpentine.ks min6rauxet les texturessont €partis par rappoft a deszones de cisaillementdans la partie inteme du massif de serpentinitede fagontelle que le degr6de recriitallisation et de remplacementaugmente vers ces zones.En gros,cette rdpartitiotr tdmoiglerait d\in m6tamorphismed0 e I'infiltration d'unephase fluide externehors d'6quilibreavec la serpentinite.Les.zones de cisaillement ont servi de conduits.La recristallisationet le remplacementde la lizardite form6e aux d6pensde lblivine et de celle form6e formes de aux d6pensde I'enstatiteont d'abord progress6ird6pendarnment, de telle sorb que la com^positiol9T di1"fgl se.p"ntitt" 6taientrdgies par la compositiondu min6ral prdcurseur.Plus tard, les rapportsF&*/:s4g, CrlAl et Fe3+/(Mg+ Si) et |a distribution {u bore entre la serpentinequi remplaceI'olivine et celle qui remplaceI'enstatite indiquent que 1'6quilibre6tait atteint d plus large 6chelle.Ia formation d'un assemblaged chrysotile + antigoritemarque cette transition.l,a recristallisation de la lizarfite eison remplacementpar cbrysotile + antigorite ont eu lieu i environ 250 x.25'C et e une valeur de P(H2O) inffrieure i 1 kbar. Nous nousseryons du systime MgO-SiO2-H2O@TISII) comme sch6ma pour reconstituerla temp€ratureet plut6t quune |a pressionde la serpentinisatio4parce que l) le clinocblore est stabili$6pm la d6compositionde la cbromite, serpentine,2) la magndtiteest Sirit un produit ou un r6actif dansla conversionde la lizardite i l'antigorite' indication qu'ellen'est pas essentielledans-la conversion de la lizardite d I'antigorite.D'apGs la min6ralogie,les textureset les compositionsde la lizardite et la cbrysotile,celles-ci seraient en fait despolymorphes dans le systime MSH. (Iraduit par Ia R6daction)

Mots-cl6s: serpentinite,serpentine, recristallisation, remplacemeng m6ta:norphisme par infiltration, stabilit6 des minft411, Cassiar,Colombie-Britannique.

I Presenraddress: Trinity School at River Ridge, 2300 East 88fh Street,Bloomington, Minnesota 55425-2187,U.5.4. TIIE CANADIAN MINERAI-OGIST

Ilrnopucnon structure within the Cassiar serpentinite (O'Hanley 1988, 1990) has provided a framework for the The phaserelations amongst the serpentineminerals establishmentof relations among the pseudomorphic lizardite, chrysotile, and antigorite have puzzled t€xtures,the nonpseudomorphictextures (and textures researcherssince fhe tbree minerals were recognized intermediatebetween the fwo) after olivine discussed and defined by Whittaker & Zussman(1956). Some by Wicls & Plant (LW9), the disribution of different investigators of antigorite-dominated 6stnmorphic types of oobastite",and the disnibution of lizardite, terraneshave interpretedthe phaserelations only in chrysotile,and antigorite. terms of antigorite and cbrysotile (Ev?nss/ al. 1976) and have left out lizardite, the most commonly occw- Geolocrclr Srrrnlc ring of the three minerats. Others have modeled the phaserelationships in terms of lizardiie and antigorite The site of the former Cassiarchrysotile asbestos (Caruso & Chernosky 1919) and have left out mine is localed at Cassiar, in north-central British cbrysotile. Recently, O'Hanley (1987) and O'Hanley Columbia. The deposit occurs within the Cassiar et al. (1989a)have employed chemographicanalysis serpentinite, which is located at the base of the using dual networks to develop possible solutions to Sylvesler allochthon. The geology of the allochthon the serpentinephase-relations problem, using all three hasbeen described by Gabrielse(1963), Harms (1986), serpentineminerals and other relevantphases. O'Hanley(1990), and Nelson & Bradford(1993). The Deposits of chrysotile asbestosin serpentinized geology of the Cassiar mine has been describedby peridotites contain a variety of textures made up of Gabrielse (1960), O'Hanley & Wicks (1987) and various combinations of the serpentine minerals O'Hanley(1988, 1990). The allochthonis composedof lizardite, chrysotile,and antigorite (Wicks & Whittaker fault-boundedpackages ofisland-arc andoceanic rocks 1977).Thus, fhey form a naturallaboratory in which to and has beenemplaced on top of late he-Cambrian to study the phase-relationsproblem. The Cassiarserpen- Devonian pladorrn-type sedimentaryrocks. Emplace- tinite, in north-cenfral British Columbi4 is an ideal ment of the allochthonoccurred between late Triassic body for this type of study. It contains all the com- and early Cretaceoustimes, but faults within the plexities and variationsof chrysotileasbestos deposirs, allochthon were active during the Permian (Harms but it is small saeughto map in detail in a reasonable 1986). The postemplacementhistory of this part of period of time. The mine was in productionduring the British Columbia is dominated by the intrusion of period of the researchproject (1987-1988), and granodioritic to granitic plutons, and by dextral the company, Cassiar Mining Corporation, provided transcurrent faults that have dismembered the accessto the mining records.This meantthat the rock allochthonousand accretedterranes (Gabrielse 1985). already mined could be included in the analysis, The Cassiarserpentinite is enclosedby argillite and 6nking our study more comFlete. chert on its footwall, andby argillite and greenstoneon The variety of serpentine textures in asbestos its hangingwail (Fig. 1). The serpentiniteis dissected deposits creates two initial impressions: first, the by severalshear zones, of which the most imForlantare serpentineminerals and texturesseem to be randomly the 45o shearand the 70o shear.The 45" sheardips 70o disfributed with respect to geological structure, and east but contains zones of schistoseserpentine and second, that serpentinization is a disequilibrium talc-magnetitethat dip 45o east(hence its name).The processin which local equilibrium cannotbe applied. 70o sheardips 70" to the northeast. However, this is not the case. The textures and The ore zone, defined as serpentinite containing minerals are syslematicallydistibuted with respectto more than 3Vochrysotie cross-fiberasbestos veins by the geological structure, and record the geological volume, is located in the center of the serpentinite, processesthat prbducedthem. The mineralsalso reflect sfucturally aboveand spatiallyassociated with the 45o not only local equilibri-um,but equilibrium at a larger and 70' shears@g. 1). There are virtually no veins of scale,for example,between Uzardtte after olivine and chrysotile asbestosin the hanging-wall serpentinite, lizardite after enstatiteQeferred to as "bastite"). Thus, whereasnon-ore veins of magnetite-bearingcbrysotile it is possible to define zones consisting of distinct asbestos,are found in several areas in the footwall assemblages of serpentine minerals serpentinite, 'lsoregimes" [called southwest of the ore zone. A model by Laurent (1980)1,upon which petro- describingthe origin of theseand ofler asbestosveins logical interpretationscan be based. hasbeen proposed by OTlanley (1988),who identifled It is the principal goal of this paperto establishthe two episodesof vein formation at Cassiar.This inter- conditionsof formation of eachof the tbree serpentine pretation is basedon vein mineralogy, and vein-vein minerals tbrough a detailed analysis of the Cassiar and vein-matrix cross-cuttingrelationships. The origin chrysotile asbestosmine. Most of the textures at of the veins is associatedwith displacementalong the Cassiar correspondto those describedby Wicks & 45oand 70o shear zones (O'Hanley 1988). Whittaker (1977), but someundocumenled variations Zones of metasomaticalteration betweenserpenti- have been observed. The detailed mapping of the nite and the argillite, cherl and greenstoneare present CONDITIONSOFSERPENTINTa:IION,CASSIAR,B.C. 755

I Serpentne with clinochlore O $rpenfine wlthout clinochlor€ - llnss dMding lsoreglmes sare"' t ..rt

d#\;'e lrnsr

Vancowor

Frc. 1. Geologicalskerch-map of the cassiar asbestosahg' Thg hanging-wallserpentinite GIW Srp) is to the east,and the footwall serpentinite(FW Srp) is to the west of the or" Country rocks consist of Mississippianto Pennsylvanian(MP) chet and argilite"on6. in the footwall, and Mississippian(m4) greenstonein the fuangingwall' The 'volume,, ot" definedby the presenceof more than 37oasbestos fiber veins by is in the"on", center of the i"rp"ritioit", bo*A"d on the southby the 70oshear and the west by the 45o shear.Bold lines divide the serpentiniteinto zones chancteized by either specific ass€tmblagesof serpentineminerals or specifictextures ("isoregimes"). These ..isoregtrireir"include: type-l lizardite hourglass(IlLz) and type-2 lizardite hourglass - - CfZlz) in Oe footwall serpentinites,lizardite (Lz) chrysotile (Ctt) antigorite-(Atg) interlocking texturesin and adjacentto the ore zone, lizardite mesh-textures(Lz) in fts hanging-wall serpentinite, and antigorite linterpenetrating textures (Atg) in ft" naUtp*-** zone of alteration (HW AZ). Sarnplescontaining clinocblore are aistingtJsned from those that do not; note there is no relationship between the serpentlne"isoregimes" and the distribution of clinochlore. Serpentine"isoregimes" are not c@xtensivewith the ore zonebecause the ore zoneis definedby the presence of chrysotile asbestosveins, not by texture and mineralogy.The hanging-wall zone of alteration(HW Aa formed throughreaction between serpentinite and greenstone; it occurs at the eastemmargin of the serpentinite.Sample numbers (i.e., C20, C56, etc.) refer to thosementioned either in the text or in the tables. at both the footwall and the hanging-wall contactsof clinozoisite, and minor tremolite after greenstone the serpentinite.The hangng-wall zone of alteration (Gabrielse 1963, O'Hanley et al. 1992)-The footwall (Frg. 1) is more extensivelydeveloped and consistsof zone of alteration (not shown on Fig. 1) consistsof antigorite formed from lizardite serpentinite,nepbrile talc--carbonatet pyrite after serpentinite;it was not jade formed from antigorite serpentinite,and quartz, includedin this study. 756 TIIE CANADIAN MINERALOGIST

In spite of completeserpentinization, the protolith tively the distribution of B in severalsamples from the can be recognized,using the criteria of Wicks (1984), Cassiar serpentinite (Carpenter 1.972,Higglns et al. as having been a harzburgite tectonite with proto- 1984). granular textures,but both schistosityand kink bands Mineral compositionswere determinedby electron arc presentin some pseudomorphicallyserpentinized microprobe.Analyses were caried out using a JEOL harzburgire(Wicls & O'Hanley1988). In handsample, 8600 Superprobehoused at the University of Westem the serpentinitecan be divided into three types based Ontario. Operatingconditions were 15 kV and l0 nA on its color and texture. The ore zone consists of a with o'glassy" a counting time of 20 seconds.Natural minerals ydow-green, to slightly grainy serpentine wereused as standards. with centimeter-sizedgrains of oxide minerals. The The techniques and results of Mdssbauer spec- serpentinitebetween the ore zoneand the footwall zone troscopy(O'Hanley & Dyar 1993),180/160 and IIID of alleration (FW Srp; Fig. l) and between the ore analyses(Kyser et al., in prep.) on serpentineminerals, zone and the hanging-wall zone of alteration (HW and studies on fluid inclusions found in several Srp;Fig. 1) consistsofablack-veined, green,..glassy,' minerals in the associatedrodingite (O'Hanley et al. serpentine.The hanging-wall zone of alteration (HW 1992) ate reportedelsewhere. The originat serpentine AZ; Fig. 1) consistsof a grey-greensugary serpenti- that formed at the expenseof olivine was a lizardite nite. mesh texture; it has undergonea complex series of A minimum age of serpentinizationcan be estab- recrystallization and replacement reactions, to be lished from geological observationsand radiometric documentedbelow. dating. East-west-strikingnormal faults offset textural patterns of the serpentinein both the ore zone and TBrrunrs the hanging-wall zone of alteration, indicating that serpentinizationpredates the normal faults. Severalof Pseudomorphicte tures afier olivine these normal faults contain undeformedlamprophyre dikes, with ages from 73 to 1"10Ma (Nelson & L\zardlte-LT mesh and hourglass textures, as Bradford 1993). These dates indica0e that normal defined by Wicks & Whittaker (L977), are subdivided faulting, and thus serpentinization,cannot be younger on the basisof observationsreported at Cassiar.These than 73 Ma. different textures, particularly of the hourglass An independentconsftaint on the minimum aee textures,forrn a record of the interactionof fluids with of serpentinizationcomes from an Ar/Ar cooling ug"-of the serpentinite(O'Hanley et at. 1989b).Thus, they are 94 Ma for a Cr-bearingphlogopite taken fromthe Road not simply of academicinterest, but provide the key River Group,in the footwall of the Cassiarserpentinite. information necessaryto understand this complex The rock from which the phlogopitewas taken crossed processof serpentinizationand recrystallizaliea. the 300"C isofherm at 94 Ma (Nelson & Bradford Lizudrte-lThourglass lexturescan be divided into 1993).As serpentinerecrystallization and replacement type-l and type-2 on the basis of extinction patterns at Cassiaroccurred at 250 *,25.C (O'Hanleyet al. within the mesh cells [see Wicks et al. (1977) for L992), recrystallization and replacementmust have terminologyl. In the type-l texture, extinction sweeps occurredat or nearthis time. continuouslyacross the meshcell and is asymptoticto sectorboundaries. In the type-2 texture, extinction is E)(PsRn/Gr.nALTEctnuqurs discontinuous,owing to the presenceof coarser crystallites in the mesh cell, and is not asymptoticto The terminology of Mercier & Nicolas (1975) is sector boundaries. The type-2 howglass texture used for peridotite textures, and that of Wicks & replacesthe type-l texture (Fig. 2a). Mesh textures Whittaker (1977), as modified by Wicks & O'Hanley also can be divided into types L and 2 based on (1988), is usedfor serpentinitetextures. A microbeam the occurrence,in the mesh rims" of the same X-ray camerawas usedto identify the mineralsiz sifa extinction characteristicsas observedin the hourglass in thin section,following the proceduresof Wicls & textures. Zussman(1,975), so that the texturescould be relatedto Serpentinitescomposed of type-l hourglasstexture specific minerals. The X-ray-diffraction experiments also contain cbrysotile + magTetiteveins that have utilized Ni-filtered Cu radiation during runs of 8 hours been pseudomorphicallyreplaced by lizardite-lT under vacuum. A total of 58 microbeamfihns were contemporaneouslywith the lizardwlT in the type-l obtained from 16 samples.Whole-rock analyses hourglasstexture @g. 2b). In the normal sequenceof were obtained from X-Ray Assay Laboratories, alteration,olivine is replacedby lizardite, followed by Toronto,using X-ray fluorescencespecfioscopy. Fused the fonnation of chrysotile veins. The presenceof beads were used for major elements, and powder l:zardtte pseudomorphsafter chrysotile veins in the pellets were usedfor minor elements.HrO+, HrO- and type-l lizardite hourglass texture leads to two FeO were determined from wet-chemical methods. inferences: 1) the type-l lizardite hourglass texture Alpha-track mapping was used to determinequalita- formed from a pre-existingserpentinite (Prichard 1979, CONDMONS OF SERPENTINZATION. CASSIAR. B.C. 757

O'Hanley 1991), and 2) the reaction lizardite = Terturesafier enstatite chrysotile was reversed.Brucite was not observedin either thin section or microbeam studies of samples At Cassiar, the serpentine pseudomorphs after from any part of the serpentinite,with the exceptionof enstatite("bastite") have a continuousrange oftextures one sample of "bastite" (see Textures after enstatite, and mineralogy, and presentan opportunity to solve below). someof the problems(such as the implications of the '"bastite"texture) noted by Wicks & Whittaker (1977) Nonpseud.omorphic texture s afi er olivine and Wicks & Plant (1979). The textural types of '"bastite" have been named, in order of increasing There are two types of serrateveins at Cassiar: 1) degreeof recrystallizationouniform, domainal,patchy, lserpentine serrateveins composedof chrysotiTe-2M., shaggy,and indistinct. These are illustrated and dis- and previously documentedby Wicks & Whittaker cussedby Wicks & O'Hanley(1988). Uniform and (1977), and 2) cr-serpentineserrate veins composedof domainal types of "bastite' consist of lnardrte-lT. hzafirte-lT. Chrysotile serrate veins occur in inter- Some examplesof domainalobastiteo' in the hanging- locking textures(Wicks & Whittaker L977)n the ore wall serpentinitecontain magnetite.Patchy "bastite" zone,where they are associatedwith antigoriteand cut consistsof lizardite-l?that wasreplaced by chrysotile- by veins of chrysotile asbestos.Serrate veins of 2M"1. Shaggyand indistinct types of "bastite" consist lizardite are present in tansitional textures (defined of lizardite-12, chrysotile-2M"1, antigorite, and below) associatedwith ribbon veins or serrateveins of clinochlore. Brucite was identified by microbeam cbrysotilelocaled adjacent to, but outside,the ore zone. camerain onegrain of patchy'"bastite", associated with Serrate veins of [zardite recrystallize to form lizardite and antigorite. chrysotileand antigorite. An unusual texture, characteristic of chrysotile Relntionshipbetween textares after olivine asbestosdeposits (Wicks & Whittaker 1977),seems to and thoseafter enstatite involve serpentine pseudomorphic after olivine in planelight but appearingas a featurelessfield of nearly T?rerelationship between the texturesformed after isotropic lizardite-ll with no mesh cells in crossed olivine and thoseformed after enstatiteis obscured"to nicols (Wicks & O'Hanley1988: Figs. 20c and d). In the casualobserver, by the diversity and complexity of somesamples, this texture is associatedwith lizardite- serpentinetextures. However, detailed observations 1I serrateveins. lizardite-lI meshtexture and ribbon from 93 thin sections clarify this relationship. A texturesowhich all pass into a lizardite + antigorite histogmm (Frg. 3) relating each type of "bastile" to interlocking texture. This texture seems to be an the serpentinetexture after olivine that surroundsthe incipient interlocking textureotansitional to a firlly '"bastite"shows, in all but two examples,that there is developedinterlocking texture. only one serpentinetexture after olivine surroundinga X-ray studiesand observationsin both incident and given grain of "bastite". reflected light (see Wicks & Plaat L979) show that Uniform "bastiteo'comprises427o of "bastite" grains chrysotile+ antigoritecoexist without clinochlorein an associatedwith hourglasstextures, but only ZLVoofthe interlocking texture after lizardite (Fig. 2c). "bastite" grains associatedwith interlocking textures. Clinochlore occurs with antigorite having replaced Shaggy "bastite" comprises35Vo of. "bastite" grains both lizardite hourglass(Fig. 2d) and lizardite inter- associatedwith interlocking textures,but only l7%oof locking textures. Clinochlore without coexisting the "bastite" grains with an hourglass texture. antigorite occurs in lizardite '"bastite", and as a rim Domainal and patchy "bastite" occur in both aroundrelict ferrian chromiteand chromite (Fig. 2e). textures, witl domainal "bastite" slightly more Lizardite + magnetiteveins of variable width with common in hourglasstextures, and patchy '"bastite" qu$patg vein-walls (Fig. 2e) cut the antigorite t slightly more commonin interlocking textures. clinochlore interlocking texture and are in tum cut by Recrystallizationof serpentinizedolivine increases veins of magnetite-ftee chrysotile asbestos.As the from mesh texture through to interpenetratingtexture antigorite t clinochlore texture has formed from (Wicks & Whittaker 1977),e.9., hourglass textures are lizardite-bearing textures, the existence of the less strongly recrystallizedthan interlocking textures. lizardite-magnetite veins cutting the antigorite t Based on the correspondencebetween "bastite" clinochlore texture indicaies that a reaction involving textures and the texture of serpentinethat replaced the conversionof lizardite to antigorite t clinochlore olivine adjacentto the "bastite", the extentofrecrystal- was reversed.Magnesite - lizardite - magnetiteveins lization of "bastite" increases from uniform to cut all other textures and veins and are located indistinct "bastite". Flowever,during the recrystalliza- throughoutthe serpentinite.The magnesite- lizardite - tion of patchy oobastite"ochrysotiTe-2M"1 formed, prior magnetile veins are physically deformed, indicating to antigorite, whereas in interlocking textures a.fter that deformationcontinued after textural development olivine, lizardite recrystallized directly to form of the serpentinitehad ceased. antigorite. In addition, the association of uniform 758 THE CANADIAN MINERALOGIST

"bastite" with hourglasstexture, and their presencein altered samFles,indicating either that 1) lizardile is interlocking textures, suggestthat "bastite'?textures stablewith cbrysotileand antigorite,or 2) the reactions become modified at a different rate than serpentine did not go to completion.We prefer the fonner alter- textures after olivine. Thus, there is a generalcorre- native on the basis of complete destruction of tle spondencebetween serpentinetextures after olivine lizardite hourglasstexture in the most strongly altered and "bastite" textures, but in detail, serpentinized samples.ff lizardite was not stablewith chrysotileand olivine and'"bastite"evolved somewhat independently. antigorite, then lizardite would have been replaced when the hourglass texture was overprinted by Spatial distribution of serpenrtncminerals chrysotile and antigorite. As lizardite persistsin the and textures interlocking texture with cbrysotile and antigorite,we infer that it is stable(with a slightly modified compo- The textural complexity of the Cassiarserpentinite sition: seebelow in Table 2). is typical of chrysotile asbestosdeposits (Wicks & In the southwest€mcorner of the mine, the serpen- Whittaker 1977).However, it is now possibleto define tinite is composedonly of q?e-l lizardite hourglass the textural evolution of the Cassiarserpentinite on the texture after olivine, uniform and domainal oobastite". basis of geological structure determinedby detailed and accessorypyrrhotite and complexferrian chromite mapping. Mineral "isoregimes" for chrysotile and - magnetite- serpentinegrains (zone labeled TlLz; antigoritehave been plotted on a geologicalmap of the Fig. l). Type-l lizardite hourglasstexture has been Cassiar serpentinite,along with a boundary between recrysttllized to type-2lizardite hourglasstexture, and hourglass and transitional textures [the (fype-l lizardite serrate and ribbon veins. This marks the Lz)-{type-2 Lz) boundary;Fig. 1l basedon their first initiation of the change from hourglass (pseudo- appearatrce.Lizardrte persists in the most strongly morphic) to interlocking (nonpseudomorphic)texture CONDITTONSOF SERPENTINZATION, CASSIAR, B.C. 759

Frc. 2. Photomicrographs:all scalebars in millimeters. a) Tlpe-l lizardite hourglasstexture being partly recrystallizedto form typ-2 hzardrtu hourglasstexture, based on presenceof crystallites (L) ftat disrupt extinction patt€m (sample C56). b) Recrystalizedcbrysotile vein (rv), now lizardite-lf, with magnetite(C55). c) Antigorite (A) and chrysotile 2M.t (C) n lizardite + chrysotile+ antigoriteinterlocking texture, in reflectedlight (C20). d) Antigorite + clinochlore(Cl) after lizardite hourglasstexture (L) (C7l). e) Chrysotile asbestosveins (C) cut lizardite-magnetiteveins (L) that cut a:rtigorite-chlorite interlocking texture, with clinochlore (Cl) rinming relict chromite (S). (C112). f) Complex ferrian chromite - serpentine grain dmmed by magnetite,in reflected ligbt (C20). g) and h) Alpha-track maps illustrating variations in boron content befween"bastite" (B) and serpentineafter olivine (S) basedon a grey scale.Darker-colored minerals (a9., "bastite") have higher concenhationsof boron, whereaslvhite-colored minerals (ag., oxides)have fittle to none.In g), "bastite' contains more boron than does serpentineafter olivine (C56), whereasin h), serpentinerecrystallization has produceda uniform distribution of boron (C50).

(zonelabeled type-2Lz; Fig. 1). magnetitegains are much more abundant.No sulfide Justwest of the 45o shearand southof the 70o shear minerals occru in thin sections of these samples. (Ctl and Atg zones; Fig. 1), lizardite t antigorite Although asbestosveins are presenq they did not * clinochlore interlocking textures and lizardite * constituteore owing to the springinessof the asbestos, chrysotilet antigoritepatchy "bastite", associatedwith which resulted in burst bags of milled fiber and uniform and domainal "bastite", replace the type-2 unfavorablephysical properties. lizardile hourglasstexture and the serrateveins and The ore zonelies north and eastofthese shearzones ribbon veins of lizardite. The association ferrian and consists of a lizardite t chrysotile interlocking chromite- magnetite- serpentineis rarely present and texture with chrysotile serrale veins t antigorite t 760 THE CANADIAN MINERALOGIST

Bastite Textures

ffi uniform ra ffi domainal .Ea q2 200 ffi latchv (H shaggy lio fi

H indistinct P I

mesh hrgls inlk inpt olivine texture Flc. 3. Correspondencebetween serpentine textures after olivine and thoseafter enstatite. A plot of five "bastite" texturesrelsrlJ four serpentinetextures after olivine (mesh: meshtexture, hrgls: hourglass,inlk: interlocking,inpl interpenetrating).Two grainsof "bastite" are surroundedby both pseudomorphicand nonpseudomorphic textures; both were plotted twice.

clinochlore t magnetitet headewoodite.Clinochlore divergence of the antigorite o'isoregime" and manflessome relict grains of chromite.Relict lizardite clinochlore-bearingsamples. In contrastto antigorile, pseudomorphictextures and relict ferrian chromite - clinochlore is easily identified in thin section by its magnetite- serpentinegrains are presentin the ore interferencecolor, so that the presenceof antigorite 'oBastite" zone. variesfrom domainalto indistincl with without clinochloreis a soundinlerpretation. the indistinct "bastite" consisting of lizardite t The hanging-wall serpentinite,east of the ore zone, antigorite+ clinochlore. consistsof meshtextures with lizardile and chrysotile There appearsto be a continuouszone of antigorite mesh-rimsand lizardite hourglassand isotropic mesh- along the 45o shear (Fig. 1), but a great deal of centers. Uniform hzardtte-LT"domainal hzafitte-LT microbeam X-ray diffraction work will be required (some with magnetite) and patchy hzardrte-LT - to delineatefirmly the presenceof this zone.Antigorite chrysotile-2M"1oobastite" also are present.Cobalt-rich is dif&cult to recognize in interlocking textures in pentlanditeis the sulfide mineral (seebelow, Table 6, thin section; it is thus possible that the samplesof 6-7 to 6-1.1).Asbestos veins are absent. interlocking texture that were not studied by X-ray An antigorite t pentlanditeinterpenetrating texfure diffraction also containantigorite. occurs in the hanging-wall contact-alterationzone. It The distribution of clinochlore (for which an formed throughthe recrystallizationof lizardite type-l ooisoregime" is not shown on Fig. 1) is different from mesh texture (Wicks & O'Hanley 1988, Fig. 20d). that of antigorite. In particular, the orientation of the Asbestosveins are absent. trend defined by samples containing clinochlore is Recrystallization tlrough the various textures of 'oisoiegi4es" oblique to the serpentine ngar the 70o lizardite to chrysofile and antigorite increasesas the shear and perpendicular to that of the chrysotile center of the serpentinitebody is approachedfrom 'lsoregime" at the southern end of the 45o shear either the east or the west. The change in mineral (Frg. 1). In addition, antigorite was found in several assemblagescorresponds to the differences in hand samples without clnochlore, as indicated by the samples.The yellow-green serpentinitethat hosts the CONDITIONSOF SERPENTINZATION. CASSIAR. B.C. 761 ore zone has a hansitional to interlocking texture Mnmnar Crnmsrnv consisting of lizardile, chrysotile and antigorite t clinochlore t heazlewoodite.The grains of "bastite" Electron-microprobedata for the silicate minerals are shaggy to indistinct and consist of lizardite, from the Cassiarserpentinite are shown in Tables 2 cbrysotile, antigorite t clinochlore with relict oxides. tbrough6. Rock-forming serpentinein samplesC50 to In confrast the black-veined,green serpentinite outside C56, Cl39 and rwo samplesof chrysotile asbestos the ore zone consistsof lizardite hourglasstextures in (C167 and C200) were analyzedfor Fe3+lFe2+values the footwall serpentiniteand lizardite mesh and hour- by Mdssbauerspectroscopy from powders obtained glass textues in the hanging-wall serpentinite.Both from hand sarnples(O'Hanley & Dyar 1993). Even contain magnetiteveins t pynhotite r pentlandite+ though observationsof thin sections were used to relict oxides.The'"bastite" grains are uniform to patchy choosethe locationfrom which to take the samples,the and consistof lizardite t chrysotile+ magnetite. Mdssbauervalues must be consideredas representing The grey-greensugary serpentinitefu fte fusnging- whole-rock valuesbecause the analyzedpowders may wall alteration zone is composed of antigorite t contain serpentineafter both olivine and enstatite.In lizardite interpenetating texture r pentlanditet mag- addition, antigorite is presentin samplesC50 to C52. netite. As the presenceof Fe3+ cannot be ignored, the microprobe analysesfor both serpentineafter olivine WHoLe-Rocr Col{posrrroNs (Iables 2 and 3) and after enstatite (Table 4) were correctedon the basisof the Miissbauerdata. In cases Whole-rockchemical analyses of samplesof the ore where no Mdssbauer data are available, mineral zone,with andwithout clinochlore.and of the footwall formulae are shownwith total iron as Fe2+. serpentinite,indicate tlat, with the exceptionof small variations in both FeO and FerO, contents,there is Lizarditeand chrysortle overall similulty in bulk composition (fable 1). In sample C112, the low SiO2 and MgO totals and the Both rock-forming (lable 2, analyses2-l to 2-10) high FeO andFerO, totals canbe attributed1o a gteater andvein-forming (lable2,7-l1. to 2-14) lizarditesare abundanceof magnetite.The ore zone containsboth similar in composition.In the footwall serpentinitein less FeO and Fe2O3(and henceless total Fe) than the thesouthwest corner of themine, lizardite has lost Fe3+ footwall serpentinile. and gained Si during recrystallization from type-l

TABI,E I. REPR,BSEI\ITATIVEWHOIE.ROCK, CON{POSIIIONS OF T}IE CASSIAR SERPENNMTE

Are73,re Footwall s{rpmtinite Bspentidte withclimcfiloe witbut cliDochloe

c2a c1l2 cr21 cllg c54 c55 C56

siq 3930 34.90 39.10 39.& 38.90 3E.60 39.S 38.8 38.60 TrO2 0.04 o.04 0.o4 o.05 0.04 0.o4 0.04 0.04 0.04 AlzQ 1.15 0.98 L.t4 4.74 0.s 0.86 1.15 l.m 1.6 ctzo3 O37 4.25 o.u 0.30 0.35 o.35 027 o.n a.2l Fe2q 3.42 tt.s7 2.94 2.74 434 4.57 325 5.75 5.08 PeOs O.0r o.o1 o.0l o.01 o.ot o.o1 0.01 o.o1 0.01 FeO 0.70 3.10 o.40 030 0.80 0.80 0,4 1.20 1.00 I\{nO 0.ll o.ll o.fi o.@ o.n o.t2 0.@ 0.10 0.6 Nio a.25 4.22 o.lE o.l7 0.22 0.18 0.r5 0.19 0.19 MgP 4r.4o 36.fi) 4\@ 42.n 41.@ 41.10 4r.70 4{}.0 zl0,50 CaO 0.04 o.ol 0.01 o.0l 0.01 0.o1 0.06 0.02, a.a2 Kzo 0.or o.01 o.g2 0.02 0.o2 0.0r o.02 0.ol 0.or l'haO o.20 o.18 0.19 0.8 0.18 0.18 0.20 0.16 0.29 LO.l.* 13.40 n.60 13-$ l3.9O l3J0 1330 13.2t0 l2g 13.00

1m39 99.36 W.U [email protected] [email protected] 100.l:t tcp..M [email protected] 100.04

*LO.L: lcs on ipidm" Rb, Sr, Nb, and Ba pr€*ent i! amounts < 30 ppm. Compmitions

TABIE 2. REFRESENTATTVE ELECIRO{.MCROPROtsE DATA ON LIZARDITB AND CIIRYSSNLE

Si02 39:79 41.& &.36 40.82 39.@ 39.45 4f.34 39.& 39.7A 40.1.9 37.95 39.s1 39.41 4.W Ti02 0.05 0.00 0.m 0.04 0.@ 0,o4 0.o2 0.6 0.04 0.@ 0.6 o.o5 0.03 0.05 AlzQ 0.89 0.80 1.36 t34 0.90 a.75 0.68 0.79 t.O1 0.A7 1.52 1.64 1.06 1.18 CrZOt o28 437 0.14 0.25 0,14 0.15 0.19 0.16 033 0.29 0.20 0.72 A.n 0.68

Feo 230 1.61 3.18 332 3.51 ?.52 3.lt 3.52 330 231 3.96 3.83 2.95 2.81 A4nO 0.09 0.13 a.22 0.22 A.n 0.13 0.13 0.10 0.19 0.13 0.13 0.16 0.12 0.22 MO o.2s o.14 030 039 0.25 0.23 0.26 032 0.8 O.20 0.21 0.A 0.25 a.29 Mgo 39.& 39.U 39.90 &.r3 39.77 40.65 39.73 39.65 39.16 40.6 3a.4 39.@ 39.12 39.25 CaO 0.m 0.02 0.o5 0.05 a.gz 0.o2 0.o5 0.8 0.04 0.08 0.M o.oa o.04 0.6 Kzo o.{D o.v2 0.05 0.06 0.M 0.G 0.o4 0.6 0.03 0.08 o.sj 0.03 0.03 0.04 l&aO 0.01 0.m 0.06 0.06 a.az 0.08 0.06 0.M 0.v7 0.07 0.04 0.08 0.06 0.04

Tcral s.08 $.vt 85.62 S.68 S.90 85.00 u.6t s438 u.M 84.30 E2.55 A532 8334 U.6 Fmmula@ m 7 alomsof oxygens si 1.96 r.9 1.94 t.94 r.sz L.94 1.96 r.94 t.94 1.95 1.90 1.93 1.95 t.94 tIVlFe3+ 0.6 o.cz o.os 0.05 0.6 0.6 tlyjer o.01 0.00 0.o1 0.01 o.g2 0.00 AI 0.02 0.02 0.03 0.03 0.05 0.09 0.03 0.o1 wIl41 o02 o.o2 0.o3 0,0E 0.or o.a2 Cr 0.o1 o.o1 0.00 0.ol o.@ 0.00 0.m 0.00 0,0r o.or 0.00 0.@ 0.01 0.o2 t1tllp69+ o.u2 0.u o.05 0.06 0.06 0.06 Fe2+ o.M o.o2 0.o2 0.v2 0.03 0.03 Fe o.13 0.t4 0.14 0.10 0.17 0.16 0.12 0.11 Mn o.@ o.ol o.ol o.o1 o.0r o.0l o.01 0.@ 0.01 0.ol 0.01 0.01 0.0r o.o1 Ni 0.01 o.o1 0.o1 0.01 o.o1 0.o1 0.ol o.o1 0.01 0.01 0.o1 0,01 0.o1 0.or Mg 288 z.AL L85 2.s 2.eL 2.n 28 2.89 2.85 2.9 2.6 2.U 2"88 2.8

GiAI o.429 4.429 ftZ+ll\4s o.m9 0.005 0.fi)5 F#*rorA/ (Si+Mg) 0.0@ o.o15 0.o16

SeeFig..r fc mple locatioan,I ad 2: Uzardite-lF,inrclocking r€xftre, eample C50;3 ud e UzarOte-tf, typ*Z n nrrgtass,C5 ; 5 and6: Lizaditgll, type-l horrglass,CS; 7 md & Ueardit+lT,ulaqFr r ' intedcfing,ruErrr'E[B, tfvrD; ff ; ev ancN 1Orv; chrysc{ile-21\iii,u&]wt9zluol, iotutto"*i'ng,llrwnffing, bt13iLI rt; tfrl rya t2 Lizardite"f, h Uzardit€-nagnetitevein: 11:C?, L2: CK;13 anrd14: Lizardit+lf, type-t lourgtaa iptaces ckyecfrl6asbestm in vein,C56. C@p6itions rcpo{t€xtilt wt% qide.

(2-5, 24) ta tyry-z hourglassQ-3, 24) to interlock- I,jzardrte in the footwall serpentinite(2-13, 2-14) ing lextlrre (2-1, 21} However, the microprobe- that formedby replacementof chrysotileasbestos veins derived compositions of chrysotile Q-9, 2-10) and hasthe samecomposition as the rock-forming lizardite luardrte (2-:7,2-8) in interlocking texture in the ore occurringnext to it (2-5,24} The vein lizardite from zoneare similar, and both mineralshave compositions both the foofwall serpentinite(2-12) andfrom the ore similar to that of recrystallizedlizardite in the footwall zoneQ-ll) containsslightly moreAl andtotal Fe than serpentinite.These analysesshow that lizardite and the rock-forming ltzardrteand chrysotile after olivine. chrysotiledo not differ significantly in total Fe content, although O'Hanley & Dyar (1993) demonstrated Anrtgorite that they do differ in Fe2+lFe3+ratios. The Al content of both lizardite and chrysotile varies from 0.68 to Oxide values for antigorite were reducedto sEuc- L.36 vrt.VoAlrOr. As the parentolivine must have had tural formulae using an assumedM = l0 (whereM = a very low Alcontent (Coleman& Keith 1971),the A1 Na; A: superstructurewavelength; a: unit-cell para- in theseserpentines after olivine must have comefrom meter; Uehara& Shirozu 1985).This is a commonly the breakdownof cbromite (see oxides, Table 6) and occurring wavelengthfor antigorite and is convenient the recrystallizationof"bastite" (Table4). for comparingresults of analyses. CONDITIONSOFSERPENTINZA-TION,CASSIAR,B.C, 763

Antigorite exhibits a greatervariation in composi- from the least-recrystallizedsample (C56) tbrough tion than lizardite or chrysotile becausethere are tlvo samplesexhibiting intermediateamounts of recrystal- different occurrencesof antigoriteat Cassiar(Iable 3). lization (with increasing amountsof recrystallization Antigorite from the ore zone (analyses3-l to 3-6) has from sample C55 to C51) to the most-recrystallZed a compositionsimilar to that of lizardite from the foot- sample(C50). Arrows betweensuccessive samples in wall serpentinite(2-L to 2-6) exceptfor slightly lower Figwe 4 point in the direction of increasingamounts of Fe conlents(antigorite: 0.08 to 0.11 Fe per 7 atoms recrvstallization. of oxygen; lizardite: 0.06 to 0.15 Fe per 7 atoms of Io th" .*" of Fe2+[vlg(Fig. 4a), the points repre- oxygen). The lower Fe content of the antigorite is senting slightly less (C52) and slightly more (C50) attributedto the forrnationof magnetiteassociated with recrystallizationdo not plot on the diagonal,so that the antigoritecrystallization. location of C51, which does, is probably forttritous. Antigorite from the hanging-wallzone of alteration The CrlAl values exhibit a trend toward the diagonal (3-7 to 3-9) has much higher Fe and lower Mg with increasing recrystallization, but the points do contentsthan antigorite in the ore zone. SampleC139 not plot on the diagonal (Fig. b). The data for Fer+/ (3-7, 3-8) occurscloser to the contactbetween serpen- (Mg + Si) also exhibit a trend toward the diagonalwith tinite and greenstone(Fig. 1) than does sampleC141 increasingrecrystallization (Fig. 4c). The three points (4-9). The greater amounts of modal antigorite, representingthe most intenserecrystallization (C52 to antigorite needles replacing pentlandite and tle C50) plot near or on the diagonal. The data indicate absenceof magnetitein C139 suggesta greaterdegree that although equilibrium has not been establishedat of recrystallizationthan in C141.The higherFe content Cassiar,it was approachedwith increasingrecrystal- in C139 (comparedto that in C141) is consistentwith lization. its more intenserecrystallization. The valuesof Fe2+/i\tIg(Fig.4a), CrlAl (Fig.4b) and Fe3*(Mg + Si) (Fie. 4c) all indicate an increasein "Bastite" uniformity of composition with an increase in the extent of recrystallization and replacement.Another Grains of "bastire" (table 4) have a slightly higher Cr content than serpentineformed at the expenseof olivine (Tables2, 3), probablyreflecting the Cr conlent of the original enstatite.Lizardite in uniform (4-9, TABLE 3. REPRESBNTATIVBEIP TRON-MCROPROBBDATA ON 4-10) and domainal (4-3, 4-6) textureshas a higher ANTIOORITts Fe and lower Mg, Cr, and Al content than lizardite (4-L, 4-2, 44, 4-7, 4-A; and chrysotile (4-5) in patchy textures. Clinochlore occurs in shaggy(4-12, Si02 &m 4:7r 4O3E 40.14 n,E &3O &,iA A,5l 4ll5 zt-13) and indistinct "bastite", althoughthere is also a Tq 0.00 0.or 0.05 0.v2 0.92 004 0.06 0.06 0.@ At1p3 1$ l.r2 L94 154 6.El l.Tl 0.66 o.8it O.n chlorite-like minslal in parchy "bastite" (thin-section cr2q o.o4 ofr 0.15 0.1r 0.70 059 a4l 0.69 036 observation)that only contains 7 vtt.Vo N2q @-9} ItO L99 2(a 2.y 29 L10 26.t 631 5.92 !.4 Clinochlore containsthe highest concentrationsof Cr tvtuO o.19 0.13 0.16 0.19 0.t6 0.I9 033 ofr 0.25 t{o o.t7 0.16 0.8 0n ozt o o2t o)A o.17 of the silicatephases (4-12, 4-13). Mgo 949 &25 39.g2 3921 37.78 40.&) XAg X& *6 co 0.04 0.o2 0,@ 0.04 0.06 0.05 0,6 0.05 0M Cornparisonbetween lizardite afier olivine and K2o ool g.g2 0.6 0,04 0.6 0.04 0.04 0.06 0.a lb?o 0.06 0.04 0.05 0.05 0.s7 0.@ o(B 0.o5 0.o2 lizardit e arte r en"rtatit e 8,n uJ4 6 U.B a'.A 86.29 97.€ CrJ5 E4.6E The textural mineralogical evidence suggests and Fo@ula bed q 6.E aeE of oxlgeo# that the serpentinesafter olivine and after enstatite evolved somewhat independently of each other. si 1.87 1.92 l.ta L94 t:74 l.t6 1,99 1. t.94 Ftbt+ 0.04 0.q| 0.@ oo4 o.(rJ o.92, Mineral compositionscan be used to test this con- BvlAt 0.06 0.03 0.05 0.o2 clusion by establishing the presenceor absenceof AI o.l9 0.u o.02 equilibrium betweenserpentine derived from different Ml4 0:00 0.00 0.m ao2 0M, o.@ can be examinedby Cr 0.@ o.@ 0.m o@ 0,01 0.ol 0.0! 0.01 0.01 hosts. For example,equilibrium twhlt o.0B o.o2 o.Gl 0.08 0.u 0.G calculatingcation ratios, $uchas Fe2+lN4g,for coexist- Fe+ 0.04 0.G o.g2 0,42 0.t9 0.18 R o.oa 0,10 0.13 ing lizardite after olivine and that after enstatite.If Mi 0.08 0.01 0.01 0.01 0.ol o.0r o.ol 0.ol 0.ol plot Ni o.0l o.o1 0.ol o0l o.0l o.0l o,ol 0.ol o.ot chemicalequilibrium exists,then on a of Fe2+/Mg Mg L70 2.9 ln 2.61 L62 ZEI Z't 23 26a in lizardite after olivine versus F&*tMg in lizardite after enstatite, the data point$ should plot on the F*i/Mg 0.01 0.01 0.01 o.0r 0.G 0.07

diagonal. ! Se 6 tuFdi8ldm d 6.E @8 d 6yg€a F ft.qls SF F|SF3 I tu 8a!ryle The valuesofFe2+[VIg (Fig. 4a), Cr/Al (Fig.4b) and tocarl@ OF @ f ald 2 dgdfq idFrod@ eEdEs, saEpls @ 3 d 4: Fel(Mg + Si) (Fig.4c) for samplesC50 to C56, all sdgoElto, ffidsrs g1l 5 Ed 6; rdgpatb, ffisfh& C113. Eqngiig+tr altEntl@ @ 7 ad 8: dgolib, hcsp@' Cl:|q I edgGite' @pa€cEltu8; from the footwall serpentinite,represent the nansition cl4l. 764

TABI.E 4. REPRF.SENTATIVEELECTRON.M(ROPROBE DATA ON NBASTITEN

l0 l1 L2 13 siq 38.89 39.& 39.67 3936 39,57 39.59 39.9t 40.00 40.m &37 36.10 32.M 3L.6 riq o.00 0.4 o.05 0.02 0.tr2 0.05 0.M 0.0a 0.01 0.03 0.05 0.o7 0.0? AlzQ 0.86 1.29 1.50 0.84 1.06 r.37 1.22 138 1.29 1.07 7.@ 13"97 r3.y CrZC}: o.57 4.52 0.52 0.60 0.60 0.91 0.60 0.7S 1.o4 0.84 r.g7 2.6t 2.70 Fbo 1.53 2.LO 2.57 1.61 1.68 2.95 2.m 231 2.M 231 2.58 2.% 3.05 hInO o.25 0.22 A.M 0.20 0.14 A.M O.20 0.30 0,t7 0.15 0.18 0.18 0.13 Nio o.2L 0.26 0.31 A.n 0.26 0.a o.n 0.25 0.25 0.?s o,?a o.ztt. o31 Mgo 39.72 39.t2 39.31 39.76 40.13 38.84 39.7a 39.80 39.n 39,X 36.9 39.W 34.11 cho o.vz 0.92 o.07 0.04 0.m o.03 0.05 0.05 0.ol 0.o4 0.04 0.08 0.09 r90 a.az 0.s2 0.03 0,u2 0.92 0.08 0.o4 0.05 o.04 0.04 0.o4 o.04 0.04 l.IaZO o.o4 0.04 0.0rl 0,13 0.@ 0.04 0.o5 0.w 0.06 0.10 a.vz 0.04 0.o7

Tod gz.Ll 8.26 Uj4 C2.75 83.8i/ U.29 U.L6 85.01 U.@ U.U 85.94 S.05 85.?1

Fqmulabased on?atoms of oxygens si 1.93 1.95 L.y3 1.94 r.94 1.94 1,.94 L.93 1,.94 L.% t.73 L.Y LY trvlp"3+ o.g2 0.08 0,04 0.03 o.a2 a.a 0.o3 0.04 o.o4 0.o4 Irvlat o.02 a.oz 0.03 o.a2 0.08 0.u2 0.03 0.03 a.m 0.00 AI 0.22 0.40 039 Ivllu o.00 0-m 0.o1 0.m 0.m o.w 0.0r o.0r 0.02 0.8 Cr 0.o1 0.01 o.01 0.01 o.01 0.u2 0.ol 0.01 0.o2 0.02 0.@ o.05 0.05 wllrd* o.uz a.o2 0.04 o.08 0.o3 0.04 0.04 0.04 0.M O,M Fsz+ a.v2 0.@ a.a a.w o.o2 0.01 0.o1 0.01 0.4 0.az Fe 0.10 0.12 0.13 Mn 0.01 0.01 0.ot 0.01 0.01 0.01 0.01 0.o1 0,01 0.01 0.01 0.01 0.01 Ni 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.o1 0.0r o.o1 0.01 0.01 Mg 2.95 zffi 2.K 2.y3 2.9r 2.u 2.8 2.K 2.8 2.8 2.& 2.43 2,6 Cr/Al 0.333 A.222 0.182 o.t43 o.m rJ+itr,tg o.o@ o.oo9 o.m9 0.w7 0.@ F#+1o6/(Mgr-Si) 0.009 0.018 0.017 0.u22 0.v26

139 2 Uzardite,lf, parchycxture, smple C$; 3: Uzardite-lT, domainal,C51; 4 Iizardits-IT, palcA[-SZ; S: ClrysoAte. potshy, 2[{s1, c52; 6, Lizardite-1?" doninal, C54; 7 and & Uzardite-lf, lof,cly, C54: 9 aad lO ilzardire-lf, uniford, C56: lL chlorite-like mineral, shaggy,C112. 12 and 13: Clinctrlore, shaggr, C112. SeeFig.l for eanple locarions. indicationof greatercompositional uniformity between The changein cationratios andin the distribution of serpentineafter olivine and that after enstatite with B between serpentine after olivine and that after increasing recrystallization and replacement is enstatitesuggests that ttre two changedindependently provided by the disnibution of boron (B). The results early in recrystallizationbut approachedsimilar values for samplesC50 to C56 indicate a continuouschange as extent of recrystallizationincreased. The similalily in B distribution with increasingrecrystallizali6l. 1Xe in compositionbetween "bastite" andmatrix serpentine contrastin B contentbetween lizardite after olivine and can be attributedto a large extent of recrystallization that after enstatitein C56, the least-recrystallized onthebasisofearlierdiscussionofserpentinetextures. (Frg. 2g), and the absenceof contrastbetween these lextuml elementsin C50, the most intenselyrecrystal- Sulfi.des lized (Frg. 2h) serpentinite,document the increasing uniformity in the distribution of B with increasing Sulfides are rare in the Cassiarserpentiniteo occur- recrystallization. ring as discrete monomineralic grains. Pentlandite CONDITIONS OF SERPENTINZATION, CASSIAR" B.C. 765

pentlanditein the hansing-wall zone of alteration;the latter mineral thus predatesthe formation of antigorite' Therefore,the variation in Co content of pentlandite, from C151(5-9 to 5-11) to C139(5-5 to 5-8), over a distance of 30 meters, also must predate antigorite. E o Pyrrhotite is associatedwith hourglasstextures in fhe El footwall serpentinite,whereas heazlewoodite (5-1 to 5-5) is associatedwith nonpseudomorphictextures in T]- $t the ore zone. E Oxil.es

Magnetiteis the most abundantoxide mineral in the Cassiarserpentinite; it is presentin virtually all rock 0.008 0.010 0.012 and vein textures.Most of the magnetitein the rocks ftz+lM91o11 contains Cr, varying from 0.48 tD 4.54 txt.VoCt2O3 (Iable 6), and its composition does not changepro- b gressivelywith degreeof recrystallization.In contrast, lessthan L wt.VoCr2O3 c54 c55, c56 .\ magnetitein the veins contains .{: (O'Hanley et al. 1992). Oxide minerals exhibit a complex pattern in most samples,as illustrated by E o.4 o a chromite-serpentine intergrowth surrounded by maguetite (Fig. 29.The cbromite cores are mantled c.52 : by magnetite or by cbromian magnetite. Results of () o.3 electron-microprobeanalyses ofthe coreswere found c51 to be unsatisfactory because serpentine inclusions produced low totals. However, the compositions of the coresare clear from the analyses,and they arepre- o.2 sentedhere because they indicatethat the composition o.1 0.3 0.4 of the cores ranges from chromite (G1) to fenian O/Atlorl chromite (6-3, 6-7) and becausethey illusffate the migration of elements during serpentinization.The c 0.03 cbromite cores lost both Al and Cr during recrystallization. E o PRoPosDREAciloNs RH-ATING 6 o.a2 Mnmar AssEIvaLAcEs + ot The serpentinitesat Cassiar arc very fine-grained 3 rocks. hevious studies of serpentinitesother than at ,9 o.ot Cassiar (Wicks & Whittaker 1977) have shown that f (r) in some serpentinites,such as hourglass-texture fl serpentinites,one can be reasonablyconfident that the rock is composedof lizardite. In other serpentinites, such as those composedof interlocking textures,the 0.00E mineralogyis complex,and detailedmicrobeam X-ray- 0.01 0.00 ditraction studiesare necessaryto identi| the vmious F"3*toy'rug+$)1qg1 serpentineminerals. At Cassiar, where interlocking texturesare so abundant,it was not feasibleto do the Frc.4. Comparisonof Fd+/1vlg(a), CrlAl (b),and Fe3*(Mg + enonnousamount of microbeamX-ray work required Si) (c) valuesin lizarditeafter olivine and lizardile after to make estimates of the mineral modes in thin enstatite,using data for samplesC50 to C56taken from sections. Therefore" the discussionsof chemical Tables2 and4. reactions that follow are necessarily qualitative becausethey are based on inferred, not mea$ured, modes. Clable 5, 5-6 to 5-11) occurs in the hanging-wall Changesin mineral assemblagesat Cassiarcan be serpentiniteand is the most abundantsulfide (but still described qualitatively by the reactions shown in much less that L vol.Vo). Antigorite has replaced "table7.The reactions(1 to 5) involvingconversion of 766 TIIE CANADIAN MINERALOGIST

TAEI.Bs. REFRESEI\NATIVE ELE TRoN.MICRoPRoBEDATA oN sULFIDEss

Cr 0.c7 0.@ 0.10 0.08 030 'u.sL0.04 0.04 0.@ 0.6 0.05 0.03 Fs 0.ll 0,10 0.o7 0.09 0.n 24.rr 4.43 n.4 N,69 n4 Ni 59.y7 9.% 60,10 58.75 52.11 n3l n.94 A,63 3132 nX4 n.q3 Crr 0.16 0.15 O.2,4. 0.@ 0.10 0,03 0,6 0.0c o.s7 0.08 0.11 Co 0.12 0.08 0.o4 0.o4 0.07 L4 l.4a L43 1.75 r.73 r.75 s 39.57 )9.62 39.& 4r.w 41.58 46.66 6.30 46,36 635 6.76 6.6

mnpte C20. 4 and 5. Headeipoodg ClI). 6 ?, and8: nendanii;, cfgS. 9, 10,and 11: penrlandit€, c141.

olivine and enstatiteto form type-l lizardite axepostu- of clinochlore seems to be superimposedonto the lated on the basis of observationsat Cassiarand from processof serpentinization.Clinochlore bearsno rela- many other serpentinites(e.g., Hostetler et al. L966, tionship to the sequenceof serpentinizationdeduced Dungan 1979, O'Hanley & Offler L992).T\e produc- from the textural pat0ernswest of the 45' shear.In the tion of brucite early in the serpentinization of ore zone, clinochlore replacedthe lizardite hourglass harzburgiteis attribuledto the reaction olivine + H,O texture as well as the lizardite interlocking texture, so = lizardite + brucite (reaction l), rather than olivinJ+ that it seemsto have formed from minerals in both enstatite* H2O = lizardite. The delayedserpentiniza- textures,rather than forrning from a transitionaltexture tion of enstatiteproduces only lizardite (reaction 2), only. The high Cr and Al content of clinochlore and releasesSiO2, which combines with brucite to [comFarethe compositionof lizardite (7-:7, 2-8) and form more lizardite (reaction3). The oxidation of Fe2* antigorite (3-5, 3-6) to clinochlore (4-12, 4-I3)j to form magnetitewith chrysotile (reaction4) and the indicatesthat the breakdownof cbromile was impor- replacementof chrysotile by lizardite in the eady- tant in the formation of clinochlore because the formed veins (reaction5) are additionalreactions that requiredAl could not have comeonly from lizardite. operateto form the footwall serpentinite. Reactions 8 to 11 are proposed to describe the The sequenceof changesfrom type-l to tyry-z formation of texturesin the ore zong; thesereactions lizardite hourglassto lizardite + antigoriteinterlocking arebasically the sameas thosethat describethe break- texturecan be describedby reactions6 and7 (fable 7), down of the type-l hourglass textures (reactions 6 in which lizardite formed from both serpentinized and 7). The absenceof talc in the pre-antigorite + olivine and enstatite, together with relict chromite, clinochlore assemblage,of brucite in the antigorite t"qystallized to new lizardite + antigorite* magnetite + clinochlore assemblage,and the presenceof talc + over a distanceof approximately80 meterswest of the magnetitewithin the 45o shear,suggest that SiO, was 45" shear(Fig. 1). added to the syslem, i.e.o on the reactant side of The transition in the ore zone from lizardite = reactions 8 and 10 Clable 7). The reaction of pent- pentlandite to chrysotile r antigorite * clinochlore landite and pyrrhotite to produce heazlewoodite(11) t heazlewooditeis complex becausethe development can be describedas (FeNi)S + 02 = ft{i$ + magnetite.

TABLE6. REPRESENIATIVEELECIRON.MCROPROBE DATA c[\ O](IDBS

'ri02 o.l7 0.10 0.10 o.20 o.t2 0.ls o.l8 0,16 0.16 0.17 0.19 AleQ 14.qt 0.08 0.28 o.01 0.17 0.20 ?.22 0.r2 a.L4 0.20 0.16 crzQ 4ti9 2.& 2@ 4.00 4.s 2.80 3135 r25 0.la 9.29 3.V7 RO a.u 95.8 65.& 8935 45.43 g).y2 53.7t n.9t y3.06 85.56 91.14 lvIgo 5.% r27 139 0.38 li3 0.?8 2.42 0.9t. O.lE O,n o.L4

p,?s ys,94 gt.99 94m slrrp tods dle io ri[cao inctud@& S.eeng. t foeange tocat@ magrnite'cZo.3: fenim crromite,cr. * ibcwian q.ie!€{ite, cs0.Saode, antr** mic!"d!", c56. 7: f€nisn chrouib, c11i|. & chrcnriaanagaetite, ct13. c *{;dler c141, 10and 11:chrcmial masNtite, Clal. CONDfiONS OF SERPENTIN6-T1ON, CASSIAR' B.C' 767

TAB[.8 7. POSTUIATED SERPEi.IflNE REACTIONS IN THE CASSIAR SERPENTINITE

hfenedfrom rhiusecrim Systen MggSiOz-H2O

Initial s€rpentinizalion

O[ + En loType-l I-z bourgla$ t€xtue & [z "basiteo

1, (mesh):Olts + H2O+ 02 =Lz+ Brc+ Met Fo + H2O=Iz+Brc 2. ("bostite") En + H2O = lz + SiOZaq) En+ H2O = lz + SiO2 3. (typ€-l hour8lass)BtG+ S|OZ+ I{2O:=La Brc+ SiO2+ H2O=L 4, (vdn)Iz,+OZ=Cd+Mgt Izs C'tl 5. (vein) Ctl : Ie Ctl=Ia,

Compsite R€ortiotr Ol+ Fo+ En + H2Q=IZ

fhotwan adjace0tto 4Y shFar

Typel La hotrglas toTypo-2 Iz, burglass to lz + Arg int€rlookilg texire 6. tyle-l lz + Chm+02 = tyF-2 L + Mgt ohegeh tgxuro 7. type-2lz + SiO2+ Q2 = Atg + MBI La+ SiO2=tr19 CmpoaiteReaceion tn+l I.c + Chm+ SiO2+ q2 = Atg + 2 Mgt Iz+SiO2 =Atg eefue

Typc2 Is hrorglass !exi['e b Ctl + Atg iaterlocking texMe

8. lz +Chm + SiQ+ Q:Atg + Cl + Mgt Ir+ SiO2=Atg 9. l.z'-C/l Iz= C'tl 1O.ts+SiO2 =Arg Is+ Siqz =Atg 11. (FeNi)S+ QZ= Ni3S2 + MCl + s2 mtepplicabls

C@potrite Rear{ion 3 Lz + Ctm + (FeNi)S + 2 SiO2 + 2 Q = 3Lz+2SiO2= 2Atg + Cd + 2 Mgt+ Ni3S2 + S2 2Atg+ Cd

Hmeinc$,au Cotrqqt-Alt4ntign 2$6€

L, mesh-rim texture to Atg iateryeretrating lexffie

12. la + SiO2 + (FoNiCo)S + Mgt = I-z+ SiO2=dtg Atg+S2+02

* Rsactionseebesd@oboerv€dchegos inmireralogy frm thinsecd@. R€actim are &matic becauseminenl modcscoullnot be estirnat€dfrm tlia rection Cmpcite reactims alebas€dotrtb smof tbindividrnl reactimlist8dfcr€athtra6iiioL *8 AtfrreviatioN Ol: olivire; Fcr fmlerite; lr: lizardite: Brc: hocit€; Mgc magnetite;Eo: enstatite;Crl: chrlnotile; Chm chromite; Cl: clinoc,blore.

Sulfidesaxe not sufficiently abundantin the samplesto D$cussIoN be more specific. The replacementof pentlanditeby heazlewooditeis typical of higttly serpentinizedrocks Phasediagramfor the serPentines (e.s.,Shiga 1987). The development of the hanging-wall contact- The changesin serpentinemineralogy at Cassiarcan alterationzone is distinct from the developmentof the be modeledto a fimt approximationusing the system ore zone. Antigorite overprints a partialy recrystal- MgO-SiO2-H2O (MSII). This simple system can be lized lizardite mesh-rimtexture. Lizardite, pentlandite, usedfor the following reasons.First, Mn, Ni, Cr andAl and magnetitewere destabilizedduring the forrnation values are either very low or similar in all three of antigorite,yielding a neady monomineraliczone of serpentineminerals, and thus were not important in antigorite(reaction 12; T able7 ). their stabilization. Second, clinochlore, which is 768 THE CANADIAN MINERALOGIST

enrichedin Cr and Al with respectto the serpentine (1989a)based on optimizedthermodynamic data of minerals, was produced by the breakdown of relict Berman (1988), as modified by Chernoskyer aL chromite and not from the breakdownof anv of the (1988),and topologicconstraints of O'Hanley(1987). serpentineminerals. Thus, it neednot be inciudedrn This diagram,modified by the addition of enstatiteto the system.Third, elecfion-microprobeand Mdssbauer the chemography,is shownin Figure5. Although topo- datafrom the serpentineminerals at Cassiarshow thar logically correct,the locations of the lizardite-bearing even though Fe is the most abundantelement substi- equilibria are only schematic because they are tuting in the se4rntine structure, the extent of its constrainedtopologically to temperaturesbelow the incorporation is minimal. The variations range from correspondingchrysotile-bearing equilibrium. The lack 0.02 to 0.06Fe3+ for Si, 0.02 to 0.06Fe3+ for Mg and of data for the end-member magnesian lizardite 0.02 to 0.04 (excludingthe antigoritefrom the zoneof precludescalculation of lizardite-bearingequilibria. hanging-wallalteration) Fe2+ for Mg. Magnetitecan be O'Hanley et al. (1989b) calculated temperatures either a reactantor a product when lizardite reactsro of serpentinization using the serpentine-magnetite form antigorite,and modal magnetite can changewhen oxygen-isotope thermometer of Wenner & Taylor lizardite recrystallizesto form a new lizardite texture (1971)on chrysotileasbestos - magnetiteveins, and (O'Hanley& Dyar 1993).Changes in modal magnetite lizardite - magnetiie from a magnesite- lizardite - are reflectedin changesin the total Fe contentas well magnetitevein. Thesesamples span the entire rangeof as in the ratios Fe3+lFe., and IvIlFe(MFe + Mg) in textures and, therefore, bracket the temperatures lizardite, suggestingthat these glanges are related to of recrystallization and replacement.The calculated changesin the activity of silica, a(SiO), rather than temperatures vary from 265 to 350'C for changesin P andT (O'Hanley& Dyar 1993). asbestos-magnetiteveins to 307"C for a Tizardrte- These observationsshow tlat it is not essential magnetitevein, and suggestthat recrystallizationwas to include magnetitein a model proposedto estimate isothermalat 300 t 50 oC.Using a correctedversion of P and T for the serpentinereactions in the Cassiar the thermometerbased on the publishedquartz-water serpentinite.However, it is an assumptionto consider fractionationfactor of Clayton et al. (7972), the mea- lizardite and chrysotile to be polymorphsin the MSH suredfractionations yield temperaturesof 250 x.25 "C. system. This assumption is not supported by the Fluid inclusionsfound in clinozoisite,grossular, and observationsat Cassiar,where lizardite has persisted diopside, the principal minerals of rodingites, which during the formation of chrysotile and antigorite, and spanthe lscrystallization event,have homogenization all three minerals have distinct compositions when temperaturesof 276 x.36'C, and salinitiesof 8 t 1.2 Mdssbauerdata are used to assign Fe2+and Fe3+to equiv.wt.VoNaCl (O'Hanleyet al. L992).The freezing coordination sites (see O'Hanley & Dyar 1993). temperatures,-30oC to -50'C, and the failure of a However, the importance of Fe3+ incoporation in clathrateto form when the inclusionsare cooledbelow chrysotile has not yet been evaluated(studies are in -100'C, indicate that the fluid is primarily of aqueous progress).The changein compositionfrom lizardite to composition,with <0.85molVo CO2.A PQI,O) of less chrysotile necessitatesttre presenceof either another than 800 bars during the recrystallizationevent was mineral phaseor ions to completetlte mass balance. determinedusing the temperatureinferred from the However,for the purposesof estimatingthe T and p of isotopic data to correct the homogenizationtempera- formation, we believe that the small elteals 6f ps tures along the appropriateisochore (O'Hanley et al. incorporationin the serpentineminerals at Cassiarhave 1992). Using the revised estimates of temperature a negligible effect on molar entopies andvolumes, and increasesthe upper estimate for P(H2O) to I kbar. can be ignoredto a first approximation. Thus, serpentine recrystallization and replacement The approximationthat lizardite and chrysotilehave occurred at 250 + 25 "C and <1 kbar. These ranges similar compositions has two implications: first, of temperatureand pressureare shown as a box on lizardite forms metastablyafter olivine, andsecond, the Figure 5. The fluid-inclusion data the absenceof system MgO-AlrOr-SiO2-H2O (MASH) used by carbonateminerals in the serpentinite,the scarcity O'Hanley et al. (L989a) is unnecessaryfor composi- of sulfide minerals,the localization of talc in the 45o tions at Cassiar.If lizardite and chrysotile are poly- shear,and the fact that all serpentinerecrysrallization morphs, then only one of them can form stably after and replacementtook place in a completelyhydrated olivine. Chrysotile is consideredstable in this system peridotite support the hypothesis that a(H2O) was for the reasonsgiven by O'Hanley (L991). Two of the approximatelyequal to 1. key equilibria in the MASH model, namely, the The serpentinereactions at Cassiarhave been modi- reactionslizardite = chrysotile + chlorite and lizardite fied to conform with the MSH systemand are listed in * talc = antigorite+ chlorite, did not operateat Cassiar, Table 7. The changes in serpentinemineralogy at indicating that the MASH model is not applicableto Cassiarcan be explainedby two stagesof serpentiniza- the phaserelations in the Cassiarserpentinite. tion in the following manner.The first stage,hydration A model of the phasediagram for serpentinein the of the peridotite, is no longer recorded at Cassim MSH systemhas been presentedby O'Hanley et al. because there is no relict olivine. and the least- coNDmoNs oF SERPENTINZATTON,CASSIAR, B.C. 769

,l I Ir +,1 I dl I rl 'J, t ll' ifi Jo { ilipi dl,fi,#/:lnl'h/J "rt tlcf,J +iS ll rii ,*${ e/ + rl :1il @/lP Fi/:* t I :t/F1 tt flq:',l I rt{I::l I I ,''ldflr ,RI I i fE:/;,/: $;!i r# i! rl 16,la/, i"! i I:ft I t) ,for' fl/ / A,,::dot 25 100 200 300 /'. 400 Temperature (oC) ftc. 5. A portion of the P(H2O)-T diagramfor the serpentineminerals lizardite (Lz), chrysotile (Ctl), and antigorite(Atg), and other relevantphases, in the systemMgO-SiO2-H2O. The otler phasesme: brucite @rc), forsterite (Fo), enstatite(En), talc (Tlc), and H2O. Stable reactionsare representedby bold solid lines, metastablereactions by long-dashedlines, doubly metastablereactions by short-dashlines, and triply metastablereactions by long- and short-dashedlines. Qrvels of meta- stability refer to assemblagesrepresenting different values for the free energy at a given point in composition space: the assemblagewith the lowest free energy is stable; tle assemblageswith the next-lowest free energy is metastable; the assemblagewith the third-lowest free energy is doubly metastable,etc.) Lizardite-bearingequilibria axe shown in topologically correct positions only. All other equilibria were calculatedusing GE@CALC@erman et al. 1986) alld the thermodynamic dataset of Berman (1988). The box representsthe estimated range of conditions of temperature and pressurefor serpentinerecrystallization and replacementat Cassiar.

recrystallized textures (hourglass textures) formed the Cassiarserpentinite prior to recrystallization,and from a previously existing serpentinite.Therefore, we isochemical recrystallization and replacementshould cannot speciS the relevant reactionson the basis of have produced chrysotile, rather than cbrysotile + data from the mine. However, the lack of serpentine antigorlte.The presenceof talc-magnetitein the shear pseudomorphsafter anthophyllite,talc, and antigorite zonessuggests that the addition of SiO2to the serpen- in the least-recrystallizedsamples suggests that olivine tinite (or SiO2 metasomatism)during recrystallization and enstatitereacted directly to lizardite pseudomorphs stabilizedchrysotile + antigorite. throughmetastable reactions below 400oC,the highest In the footwall and hanging-wall serpentinites,the temperature for the reaction forsterite + water = presenceof relict chromite in severalof the proposed lizardite + brucite in the MSH system(this estimateis reactions,the lack of equilibrium betweenthe lizardite decreasedby only 10'C if observedFe partitioningsare after olivine and that after enstatite in the least- used; Moody 1976). This is the common path of recrystallized serpentinite, and the presence of serpentinizationfor most peridotite massifs(Wicks & coexist:ngtextures in many samplesall indicate non- Whitaker 1971. equilibrium on a thin-section scale. However, local The secondstage is cbamctprizedby the operation equilibrium is presentbecause: a) one can formulate a of dominantly solid-solid reactions involving the sequenceof reactions to describe the changes in recrystallizationof lizardite to form the type-1 hour- minerslogy, composition, and the distribution of the glass texture, and its replacementby chrysotile + serpentineminerals, and b) the replacementof lizardite antigoriter lizardite interlocking texture. by chrysotile + antigorite is the stable assemblage Lizardrte was the sole serpentineminsml presentin infened from the phasediagram. 770 TTIE CANADIAN MINERALOCIST

Evafuartonof the serpentine- the P-T coordinatesof the solid-solid reactions.For magnetitethermameter and the stability of chrysotib example,greater solubility of Fe3* in chrysotile than + antigorite in the MSH system in antigorite would change tle univariant MSH equilibrium chrysotile = antigorite + brucite to the The calculated temperatures and pressures for univariant Fe3+Fe2+MSHequilibrium chrysotile = serpentinerecrystallization and replacementoverlap antigorite + brucite + magnetite.Fe3+ could stabilize the calculatedposition of the equilibrinm chrysotile = chrysotile to higher temperaturesat the expenseof antigorite + brucite (Frg. 5). As the textures suggest antigorite, possibly permitting the stable coexistence that antigorite + cbrysotile are in equilibrium, and the of Fe3+-bearingchrysotile + antigorite at temperatures comFositional data suggest tlat equilibrium was above 300oC.However, as mentionedearlier, we do closely approachedlate in the replacement,we do not not believe that the observedpartitions are sufficient believe that the conditions of the above equilibrium to significantly alter molar entropiesand volumes.Of were exceeded.This conclusion indicales excellent the three possibilities considere4 we contendthat the agleementbetween the isotope thermometerand the agre€mentbetween the temperatureestimates obtained phase-equilibrium data. However, hypothetically, from phaseequilibria and from stable-isotopefraction- partitioning of Fe into chrysotile, antigorite, and ationsis not fortuitous. brucite could affect the P-T coordinates of the equilibrium by altering the molar entropies and Causeof recrystallizationin the ore zone volumes.Thus it is necessaryto evaluatewhether the agleementis real or forhritous. Recrystallizationin the ore zone culminateswith a The above agreementcan be explained in one of lizardite t antigorite * chrysotile t chlorite * threeways. One possibility is that the calculatedphase- magnetite t heazlewooditeassemblage at T = 250 diagram is incorrect. An earlier est'mate for the t25"C, andP(H2O) <1 kbar.It is clear,on thebasis of location of the equilibrium cbrysotile = antigorite + evidence presentedabove, that recrystallization and brucite placedit at 300'C at very low pressures@vans replacementwithin the serpentiniteincrease from both et al. 1976). Solid-solid equilibria me known to be the footwall andthe hanging-wallserpentinites into the kinetically sluggish, so that it is difficult to obtain ore zone, which is spatially relatedto the 45o and 70o phase-equilibriumdata for them, especially at low shears.We eliminate classicalthermally driven meta- lemtr)eratures.In facg in their review of serpentine morphism as the cause of recrystallization for tbree phase-equilibria,Chemosky et al. (L988) included a reasons.First, the serpentiniteis chemically homo- reversalofthe aboveequilibrium as one ofthe critical geneous(Iable 1), so that thereis no siglificant change pieces of information needed to refine the phase in bulk composition to justify thermally driven diagram.Thus, the location of this equilibrium is not recrystallizationof the middle portion of the serpenti- well constraine4and the stability field for chrysotile+ nite but not its margins. Second,there is no igneous antigorite implied by the isotope temperatureswould intrusive body centered under the middle of the stjll be consistentwith the calculatedphase-diagram if serpentiniteto act as a heat source(nor did exploratory the above equilibrium werc to be shifted to higher drilling encounterone). Third, thermal metamorphism lemperafures. cannot account for the talc-magnetite assemblagein The thermodynamicdataset is a self-consistentone, the 45o shear,which requires a changein bulk-rock and models the serpentine-dehydrationequilibria well compositionfrom flat of the serpentiniteprecursor. @ermanet al. 1986).Therefore, a secondpossibility is Despite the elimination of thermally driven meta- that the calculaledphase-diagram is conect andthat the morphismas a causeofrecrystallizafien, heat is needed temperatures calculated using the serpentine- to produce recrystallizationand replacementbecause magnetite thermometer are not correct. The the solid-solid reactions are endothermic.However, serpentine-magnetitethemometer is an empirical one the enthalpy changesfor the solid-solid reactionsare basedon a comparisonof serpentinein serpentiniteto small (e.9., 17 kJ/mol for the equilibrirrn chrysotile = chlorite in pelitic schists (Wenner & Taylor 1971). antigorite + brucite; O'Hanley 1991);tlre reactionsdid Savin & Ire (1988) proposed another curve for not go to completion, so that not much heat was serpentine-water fractionation using an empirical needed. calculation basedon the type of bonds present.This An altemative cause of recrystallization is infil- curve yields similar fractionationsto those of Wenner ffation-driven metamorphism @erry 1989). In this & Taylor (1971)between 250 and350'C, which is the process,a fluid that is thermallyor chemically(or both) temperanferange ofinterest. Therefore,although both out of equilibrium with the rock producesrecrysralliza- flrryes are empirical,they are consistentin the temper- tion as the rock equilibrateswith the fluid. Infiltration- aturerange ofinterest. Thus there are no other reasons driven metamorphismexplains the texbral patterns, to doubt the thermometer. the changesin mineralogy and mineral composition, -A third possibility is that limiled incorporation of andexplains why the recrystallizationis centeredabout Fer+ into lizafirta and cbrysotile is sufficient to alter the 45o and 70o shear zones in the middle of the CONDITIONSOF SERPENTINUATION,CASSIAR, B.C. 771 serpentinite.The presenceof an externally derived specificpattems with respectto the shearzones. These fluid also accounts for changesin bulk-rock 6180 patlems indicaie the sequenfial developmentof the values (O'Hanley et al. 1989b) and for consistent various seryentine minerals and textures, and thus changesin bulk-rock compositionsduring recrystal- the relationshipsof oneto the other.No evidenceof the lization of both rodingite and serpentinite(O'Hanley initial hydrationofthe peridotiteis preserved,but some et al. L992). The shear zones in the middle of the lines of evidence for its presencecan be inferred. serpentinitewere the conduits through which a fluid Recrystallizationand replacementof serpentinewere chemicallydifferent Oigher activity of SiO) from (and complex, but ultimately resultedin the production of perhapshotter than) the serpentinite,gained accessto antigorite + chrysotile from lizardite. Chemical the serpentinile. changesaccompanying recrystallization at 250 x, 25"C, The water/rock value calculated from mineral <1 kbar indicatethat local equilibrium is attained,and assemblagesusing progress of the reaction is the equilibrium was approachedat the thin-section scale critical parameterused to determinethat infiltration- nearthe 45oand 70' shearzones. The o6servedmineral driven metamorphismwas the causeof recrystalliza- assemblagesand mineral compositionsarc consistent tion @erry 1989).Unforhrnately, we cannotcarry out with P-T estimatesfrom the MSH model for the such a calculationbecause water is neither consumed serpentinephase-diagram. However, Fe* may have an nor producedin the solid-solid reactionsof hydrated importantrole in promotingisothermal changes among solids. so that the water/rock value is moot to the lizardite, chrysotile,and antigorite. determination of whether or not infiltration meta- morphismoccurred at Cassiar.Furfhermore, we do not Acre.Iowl-mcHvH\rs know the initial 6180 value of the fluid involved in se4lentinerecrystallization, so that we cannotuse the We thank the CassiarMining Corporationfor their 6180 values of the different textures to calculate financial and logistical support during the course of the water/rock value. We also lack measuredthermo- this study. In particular, Mr. Al Burgoyne, currently dynamic data for hzatdtle, atrd this precludes of Noramco Explorations, Inc., but vice-presidentin calculation of a thermal budget.Despite theselimita- charge of exploration for Cassiar at the time, was tions, there is a great deal of evidencethat supports instrumentalin the initiation and funding of this study. infiltation-driven metamorphism.Also, the elimina- DSO thaDksthe British ColumbiaMinistry of Energy, tion of thermally dxiven metamorphism leaves Mines, and Pefioleum Resourcesfor supportingfleld infiltration-driven metamorphism as tlle preferred work in the Cassiar area, and Joann Nelson for processto accountfor the developmentof the ore zone discussionson the geology of the Cassiararea and her at Cassiar. interest in this project. FJW thanks the Natural Sciencesand Engineering Research Council of Canada Causeof recrystallizationin the hanging-wallzone for supportin the form of an operatinggrant. We also of alteration thank both Dr. Robert Bamett and Mr. David King, formerly of the University of Western Ontario, for The formationof antigoritein the hanging-wallzone assistancein obtaining the electon-microprobe data" of alteration was caused by metasomaticalteration and Dr. Michael Higgms (lniversitd du Qu6bec d involving rocks of very different compositions:the Chicoutimi) for performing the alpha-nack mapping argillites and greenstones,and the serpentinite.This experiments.Finally, we thank Associate Editor processproduced successive zones, moving from the Gordon Cressey,Jim Chisholrn and Robert F. Martin serpentinitesto the wallrocks, of antigorite, tremolite, for their commentson the manuscript. and tremolite + clinozoisite+ quartz (Gabrielse1963). There are no asbestosveins near this zone, so that Rmmmcrs deformationwas not important. The driving force for recrystallization presumably involved the chemical BmMAN,R.G. (1988):Intemaly consistenlthermodynamic differences between the two protoliths, and not datafor mineralsin the systemNqO-K2O{aG-MgO- infilnation-driven metamorphism as envisioned to FeO-Fe2O3-AI2O3-SiO2-TiO2-H2HO2.J. Petrol. 29, explain texturesin the ore zone. This processappears 445-522. to have occurredearly in the serpentinizationhistory Ewcr, M., Grum.wooo,H.J. & Bnow\r,T.H. and weldedthe serpentiniteto the hanging-wallrocks. (1986):Derivation of intemally-consistentthermodynam- This welding probably focussed successivefaulting ic alataby thetechnique of mathematicalprogramming: a along the 45o and 70o shears. reviewwith applicationto thesystem MgO-SiO2-HrO. .I. Petrol.2il,l33l-13&. ConclusroNs Canpnqw"B.S. (1972): Determination of traceconcentration The minerals and textures found in the Cassiar of boronand uranium in glassby thenuclear track tech- serpentiniteare not randomly disributed, but occur in mqlur..AruL Chem44, 600-602. 772 TIIE CANADIAN MINERALOGIST

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